The present emphasis on the conversion of coal to substitute solid and liquid fuels has lead to several alternative processes which are now being considered. Among the many processes presently being considered is the solvent refining of coal (SRC) in which coal is treated at an elevated temperature in the presence of a hydrogen donor solvent and hydrogen gas in order to remove the mineral matter, to lower the sulfur content of the coal, and to convert it into a low melting solid which can be dissolved in simple lower boiling point organic solvents. This SRC can also be upgraded through catalytic hydrogenation to produce a liquid of higher quality. Such solvent refining of coal typifies liquefaction processes adapted to improvement by the technique of this invention.
Little is known at present as to the exact mechanism by which the coal is transformed into soluble form, or of the detailed chemical structure of the soluble form, or even the parent coal. It is known that many coals are easily solubilized and for others liquefaction is more difficult. Some correlations have been made between the rank of the coal and ease of liquefaction and product yield. A somewhat better correlation has been found with the petrography of the coal. Relatively little is known about the relationship to product quality.
The initially dissolved coal (SRC) may have utility as a substitute clean fuel or boiler fuel; however, for substitute fuels of a higher quality, specifications on viscosity, melting point, ash, hydrogen, nitrogen and sulfur contents are much more stringent. Attempts to meet these specifications by operating the SRC process more severely have met with many difficulties such as low liquid yields, high hydrogen consumption, difficulty of separating unreacted residue, and excessive char formation, which often completely plugs process transfer lines and reactors.
Alternative methods of improving specifications through catalytic hydrogenation are also difficult. The problems which arise are threefold: (1) SRC components are susceptible to further condensation and may deposit as coke on catalysts used for their conversion, (2) they can also foul the catalyst by physical blockage as their size approaches the pore size of conventional catalysts, and (3) they may contain metal contaminants, and their highly polar nature (particularly because of nitrogenous and sulfur components) can lead to selective chemisorption, and thus poison the catalysts.
The precise chemical nature of the SRC is still known, generally its composition is discussed in terms of solubility. Several classifications are commonly used. These include oils which are hexane or pentane soluble, asphaltenes which are benzene soluble and asphaltols which are pyridine soluble-benzene insoluble materials. Of these the asphaltenes and asphaltols are believed to be responsible for high viscosity, solvent incompatability, and processing difficulties. Little is known about the asphaltols. It has been suggested that asphaltenes are derived from them; however, the nature of the reaction sequence has yet to be established.
In the process of converting coal to a low sulfur, low melting solid by use of recycled product fractions as solvent, several reaction steps occur. Generally coal is admixed with a suitable solvent recycle stream and hydrogen and the slurry is passed through a preheater to raise the reactants to a desired reaction temperature. For bituminous coal, the coal is substantially dissolved by the time it exits the preheater. Sub-bituminous coals can be dissolved but care must be exercised not to raise the temperature too high and thus promote charring. Exact reaction conditions and the nature of the intermediate product is, of course, affected by the solvent used.
The products exiting from the preheater are then transferred to a larger backmixed reactor where further conversion takes place to lower the heteroatom content of the dissolved coal to specification sulfur content and melting point. The linear velocity of the stream flowing through this reactor is controlled so that particulate matter of a desired size is retained in the reactor until about 40% of the reactor volume is filled. The reactor volume becomes filled (at steady state) up to about 40 vol % by solids which are produced from the coal. These solids have been shown to be catalytic for the removal of heteroatoms and the introduction of hydrogen into the coal products and solvent. The products exiting the reactor are initially separated by flash distillation to remove gases and light organic liquids. The products are further separated by filtration, centrifugation, solvent precipitation, etc.) and the filtrate is distilled to recover solvent range material (for recycle) and the final product SRC.
Properties of certain solvent components are important to the present invention. The recycle solvent derived in the process typically boils upward of about 400.degree. F. and contains phenols and condensed ring aromatics as major components admixed with lesser amounts of many types of components including those having functional groups containing nitrogen, sulfur and the like. The phenol content may be and often is reduced by external catalytic hydrogenation over a relatively active hydrogenation catalyst such as cobalt-molybdenum on alumina. That operation also generates hydrogen donors useful in the process by hydrogenation of polycyclic aromatic components such as naphthalenes, phenanthrenes, pyrenes, etc., including the alkyl substituted polycyclics.
Phenols in the solvent are useful in the initial stages of the process because of their solvent power in promoting solutions of the coal components which contribute to the product, solvent refined coal. However, as the reaction proceeds, the formation of char appears to be promoted by excessive concentration of phenols in the solvent.
It will be understood that the term "phenols" is used herein as inclusive of all aromatic components having hydroxy groups attached to ring carbon atoms, including cresols, naphthols and ring substituted hydroxy aromatics of greater number of rings.
The inorganic minerals contained in the coal also influence the course of dissolution and reaction to produce solvent refined coal. The differences in such mineral content, generally reported as "ash", can have a marked effect on the course of the process. Pyrite in the coal acts as a mild hydrogenation catalyst to regenerate hydrogen donors from polycyclic aromatics compounds. The hydrogen donors provide for hydrogen transfer to coal components to promote the desired reactions.